snprintf(): If size is zero, then snprintf() should return the size of the...

snprintf():  If size is zero, then snprintf() should return the size of the required buffer without writing anyting.  From Sami Pelkonen

libc/stdio/lib_snprintf.c

 /****************************************************************************
  * libc/stdio/lib_snprintf.c
  *
- *   Copyright (C) 2007-2009, 2011 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2007-2009, 2011, 2014 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *
  * Redistribution and use in source and binary forms, with or without
@@ -82,19 +82,41 @@
 
 int snprintf(FAR char *buf, size_t size, const char *format, ...)
 {
-  struct lib_memoutstream_s memoutstream;
+  union
+  {
+    struct lib_outstream_s    nulloutstream;
+    struct lib_memoutstream_s memoutstream;
+  } u;
+
+  FAR struct lib_outstream_s *stream;
   va_list ap;
-  int     n;
+  int n;
+
+  /* "If the value of [size] is zero on a call to snprintf(), nothing shall
+   *  be written, the number of bytes that would have been written had [size]
+   *  been sufficiently large excluding the terminating null shall be returned,
+   *  and [buf] may be a null pointer." -- opengroup.org
+   */
+
+  if (size > 0)
+    {
+      /* Initialize a memory stream to write to the buffer */
 
-  /* Initialize a memory stream to write to the buffer */
+      lib_memoutstream(&u.memoutstream, buf, size);
+      stream = &u.memoutstream.public;
+    }
+  else
+    {
+      /* Use a null stream to get the size of the buffer */
 
-  lib_memoutstream((FAR struct lib_memoutstream_s *)&memoutstream, buf, size);
+      lib_nulloutstream(&u.nulloutstream);
+      stream = &u.nulloutstream;
+    }
 
   /* Then let lib_vsprintf do the real work */
 
   va_start(ap, format);
-  n = lib_vsprintf((FAR struct lib_outstream_s *)&memoutstream.public,
-                   format, ap);
+  n = lib_vsprintf(stream, format, ap);
   va_end(ap);
   return n;
 }